Pneumatic tire

ABSTRACT

In a tire  2  according to the present invention, an outer surface  52  of a crown rib  48   e  and an outer surface  54  of a shoulder rib  48   s  form a part of a tread surface  28.  On a cross-section of the tire  2  along a plane that includes a rotation axis of the tire  2,  an outline of the outer surface  52  of the crown rib 48 e  is represented as a first arc. A center of a circle to which the first arc belongs is disposed on the equator plane. An outline of the outer surface  54  of the shoulder rib 48 s  is represented as a second arc. A ratio of a distance from a center of a chord of the second arc to the second arc relative to a length of the chord of the second arc is not less than 0.03 and not greater than 0.05.

This application claims priority on Patent Application No. 2016-209427filed in JAPAN on Oct. 26, 2016. The entire contents of this JapanesePatent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to pneumatic tires. More specifically, thepresent invention relates to pneumatic tires for small trucks, trucks,buses, and the like.

Description of the Related Art

In a tire in a running state, a tread surface slips relative to a roadsurface, and the tread becomes worn. In particular, a tire for smalltrucks, trucks, buses, and the like, that is, a heavy duty tire(including a tire for small trucks), to which a heavy load is applieddue to cargo loading or the like, is worn to a great extent.

In general, a tread of a heavy duty tire has a plurality of ribs thatextend in the circumferential direction and are aligned in the axialdirection. The ribs come into direct contact with a road surface.Therefore, by the shapes of the ribs, the extent of the wear isinfluenced. In order to improve wear resistance, the shape of the ribhas been variously studied. One example of the study is disclosed inJP2006-076359.

A tread surface of a tire has a shape that projects outward entirely.Therefore, among a plurality of ribs provided in the tread, particularlyin a rib disposed on the outer side in the axial direction, that is, ina shoulder rib, a circumferential length may be different between theinner side portion of the rib and the outer side portion of the rib.

A contact pressure tends to be higher in a portion having a longcircumferential length than in a portion having a short circumferentiallength. Therefore, in the portion having the long circumferentiallength, wear is accelerated due to high contact pressure. Meanwhile, thetire is more likely to slip relative to a road surface in the portionhaving the short circumferential length than in the portion having thelong circumferential length. Therefore, in the portion having the shortcircumferential length, wear is accelerated due to the slipping. Thus,in the shoulder rib, wear is likely to occur due to difference incircumferential length.

Reduction of the extent to which a shoulder rib is worn is attempted bythe shape of the outer surface of the shoulder rib being formed into anarc having a relatively large radius and the difference incircumferential length being thus reduced. However, in this attempt,although the extent of the wear can be reduced, the outer surface of theshoulder rib is warped during cornering, and a distribution of contactpressure in the shoulder rib may be changed. Specifically, in thecontact surface, of the shoulder rib, which contacts with a roadsurface, a contact pressure becomes high in the end portion of thecontact surface, and a contact pressure becomes low in the centerportion of the contact surface. In this case, an area of the portion inwhich the contact pressure is high is reduced, and cornering power isreduced. Reduction of cornering power causes reduction of steeringstability (responsiveness). Therefore, a driver needs to set a steeringangle so as to be large by turning a steering wheel sharply forcornering of the vehicle. Further, the large steering angle may causeincrease of a region of a portion, of the tire, which slips relative toa road surface. In this case, wear, which is to be reduced in theshoulder rib, may be accelerated.

An object of the present invention is to provide a pneumatic tire thatallows steering stability to be improved without reducing wearresistance.

SUMMARY OF THE INVENTION

A pneumatic tire according to the present invention includes a treadhaving an outer surface that is a tread surface that comes into contactwith a road surface. In the tire, the tread includes a plurality of ribsthat extend in a circumferential direction, and are aligned in an axialdirection. A rib, among the ribs, disposed at an equator plane of thetire, or near the equator plane, is a crown rib, and a rib, among theribs, disposed on an outer side in the axial direction is a shoulderrib. An outer surface of the crown rib and an outer surface of theshoulder rib form a part of the tread surface. On a cross-section of thetire along a plane that includes a rotation axis of the tire, a ratio ofa width, in the axial direction, of the tread surface relative to amaximum width of the tire is not less than 0.7 and not greater than 0.9.An outline of the outer surface of the crown rib is represented as afirst arc, and a center of a circle to which the first arc belongs isdisposed on the equator plane. An outline of the outer surface of theshoulder rib is represented as a second arc. A ratio of a distance froma center of a chord of the second arc to the second arc relative to alength of the chord of the second arc is not less than 0.03 and notgreater than 0.05.

In the pneumatic tire according to the present invention, the shape ofthe outer surface of the shoulder rib is appropriately adjusted by thelength of the chord of the second arc that represents the shape of theouter surface, and the distance from the center of the chord of thesecond arc to the second arc. In the shoulder rib, not only progress ofwear due to the difference in circumferential length but also warping ofthe outer surface during cornering as observed in a conventional tire isinhibited. In the tire, steering stability is advantageously obtained incornering without reducing wear resistance. According to the presentinvention, the pneumatic tire that allows steering stability to beimproved without reducing wear resistance can be obtained.

Preferably, in the pneumatic tire, a ratio of the distance from thecenter of the chord of the second arc to the second arc relative to themaximum width of the tire is not less than 0.005 and not greater than0.008.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a part of a pneumatic tire accordingto an embodiment of the present invention; and

FIG. 2 illustrates an outline of the tire shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention based onpreferred embodiments with reference where appropriate to theaccompanying drawing.

FIG. 1 shows a part of a cross-section of a pneumatic tire 2.Specifically, FIG. 1 shows a part of the cross-section of the tire 2along a plane that includes the rotation axis of the tire 2. In FIG. 1,the up-down direction represents the radial direction of the tire 2, theleft-right direction represents the axial direction of the tire 2, andthe direction perpendicular to the surface of the drawing sheetrepresents the circumferential direction of the tire 2. In FIG. 1, analternate long and short dash line CL represents the equator plane ofthe tire 2. The shape of the tire 2 is symmetric about the equator planeexcept for a tread pattern.

The tire 2 includes a tread 4, a pair of sidewalls 6, a pair of clinches8, a pair of beads 10, a carcass 12, a belt 14, a band 16, an innerliner 18, a pair of chafers 20, a pair of bead fillers 22, a pair offirst reinforcing fillers 24, and a pair of second reinforcing fillers26. The tire 2 is of a tubeless type. The tire 2 is mounted to a truck,a bus, or the like. The tire 2 is a heavy duty tire.

The tread 4 has a shape that projects outward in the radial direction.The tread 4 forms a tread surface 28 that comes into contact with a roadsurface. In other words, the outer surface of the tread 4 is the treadsurface 28 that comes into contact with a road surface. In FIG. 1,reference character PT represents the end of the tread surface 28.

In the tire 2, the tread 4 includes a base layer 30 and a cap layer 32.The cap layer 32 is layered over the base layer 30 from the radiallyouter side of the base layer 30. The outer surface of the cap layer 32is the tread surface 28 described above. The cap layer 32 is formed froma crosslinked rubber having excellent wear resistance, heat resistance,and grip performance. The base layer 30 is formed from a crosslinkedrubber having excellent adhesiveness.

Each sidewall 6 extends almost inward from the end of the tread 4 in theradial direction. The sidewall 6 is formed from a crosslinked rubberhaving excellent cut resistance and weather resistance. The sidewall 6prevents damage to the carcass 12.

Each clinch 8 is disposed almost inward of the sidewall 6 in the radialdirection. The clinch 8 is disposed outward of the bead 10 and thecarcass 12 in the axial direction. The clinch 8 is formed from acrosslinked rubber having excellent wear resistance. The clinch 8 comesinto contact with a flange of a rim, which is not shown.

Each bead 10 is disposed inward of the clinch 8 in the axial direction.Each bead 10 includes a core 34 and an apex 36 that extends outward fromthe core 34 in the radial direction. The core 34 is ring-shaped, andincludes a wound non-stretchable wire. A typical material of the wire issteel. The apex 36 is tapered outward in the radial direction. The apex36 is formed from a highly hard crosslinked rubber.

The carcass 12 includes a carcass ply 38. The carcass 12 of the tire 2includes two carcass plies 38, that is, a first carcass ply 40 and asecond carcass ply 42. The carcass 12 may be formed from one carcass ply38, or may be formed from three or more carcass plies 38.

In the tire 2, the first carcass ply 40 and the second carcass ply 42are each extended on and between the beads 10 on both sides, along thetread 4 and the sidewall 6. The first carcass ply 40 is turned up aroundthe core 34 from the inner side toward the outer side in the axialdirection. In the tire 2, the end of the first carcass ply 40 isdisposed outward of the end of the apex 36 in the radial direction. Inthe tire 2, the second carcass ply 42 is not turned up. The end of thesecond carcass ply 42 is disposed near the core 34 of each bead 10 inthe radial direction. The second carcass ply 42 is disposed outward ofthe first carcass ply 40.

Each carcass ply 38 includes multiple cords aligned with each other andtopping rubber, which is not shown. An absolute value of an angle ofeach cord relative to the equator plane is from 75° to 90°. In otherwords, the carcass 12 forms a radial structure. The cords are formedfrom an organic fiber.

The belt 14 is disposed inward of the tread 4 in the radial direction.The belt 14 is layered over the carcass 12. The belt 14 reinforces thecarcass 12. The belt 14 includes an inner layer 44 and an outer layer46. Each of the inner layer 44 and the outer layer 46 includes multiplecords aligned with each other and topping rubber, which is not shown.Each cord is tilted relative to the equator plane. An absolute value ofthe tilt angle is typically not less than 10° and not greater than 35°.A direction in which the cords of the inner layer 44 are tilted relativeto the equator plane is opposite to a direction in which the cords ofthe outer layer 46 are tilted relative to the equator plane. A materialof the cords is steel. The width, in the axial direction, of the belt 14is preferably not less than 0.6 times the maximum width of the tire 2.The belt 14 may include three or more layers.

The band 16 is disposed outward of the belt 14 in the radial direction.In the axial direction, the width of the band 16 is greater than thewidth of the belt 14. The band 16 includes a cord and topping rubber,which are not shown. The cord is helically wound. The band 16 has aso-called jointless structure. The cord extends substantially in thecircumferential direction. An angle of the cord relative to thecircumferential direction is not greater than 5°, and more preferablynot greater than 2°. The belt 14 is held by the cord. Therefore, liftingof the belt 14 is inhibited. The cord is formed from an organic fiber.

The inner liner 18 is disposed inward of the carcass 12. The inner liner18 is joined to the inner surface of the carcass 12. The inner liner 18is formed from a crosslinked rubber having excellent airtightness. Atypical base rubber of the inner liner 18 is anisobutylene-isoprene-rubber or halogenated isobutylene-isoprene-rubber.The inner liner 18 maintains the internal pressure of the tire 2.

Each chafer 20 is disposed near the bead 10. When the tire 2 is mountedon a rim, the chafer 20 contacts with the rim. By the contact, a portionnear the bead 10 is protected. In the present embodiment, the chafer 20is formed from a fabric and a rubber impregnated into the fabric.

Each bead filler 22 is disposed between the bead 10 and the carcass 12.The bead filler 22 is turned up around the core 34 of the bead 10 fromthe outer side toward the inner side in the axial direction. The beadfiller 22 reinforces the bead 10 portion. The bead filler 22 includesmultiple cords aligned with each other and topping rubber, which is notshown. Each cord is tilted relative to the radial direction. The cordsare formed from an organic fiber.

Each first reinforcing filler 24 extends radially outward from a portionnear the core 34 of the bead 10 along the carcass 12. The firstreinforcing filler 24 is disposed outward of the second carcass ply 42in the axial direction. The first reinforcing filler 24 reinforces thebead 10 portion. The first reinforcing filler 24 includes multiple cordsaligned with each other and topping rubber, which is not shown. Eachcord is tilted relative to the radial direction. The cords are formedfrom an organic fiber.

Each second reinforcing filler 26 extends radially outward from aportion near the core 34 of the bead 10 along the carcass 12. The secondreinforcing filler 26 is disposed between the first reinforcing filler24 and the clinch 8 in the axial direction. The second reinforcingfiller 26 covers the entirety of the first reinforcing filler 24. Thesecond reinforcing filler 26 reinforces the bead 10 portion. The secondreinforcing filler 26 includes multiple cords aligned with each otherand topping rubber, which is not shown. Each cord is tilted relative tothe radial direction. The cords are formed from an organic fiber.

As shown in FIG. 1, a plurality of ribs 48 are provided in the tread 4of the tire 2. The tread 4 includes the plurality of ribs 48. The ribs48 are aligned in the axial direction. Each rib 48 extends in thecircumferential direction. In the tire 2, the rib 48 is continuous inthe circumferential direction. The rib 48 may include multiple blocksdisposed at predetermined pitches in the circumferential direction.

In the tire 2, a main groove 50 is formed between one of the ribs 48 andanother rib 48 disposed adjacent to the one of the ribs 48. The ribs 48extend in the circumferential direction. Therefore, the main groove 50also extends in the circumferential direction.

The width and the depth of the main groove 50 influence drainage andstiffness of the tread 4. In the tire 2, from the viewpoint thatdrainage and stiffness of the tread 4 are assured, the width of the maingroove 50 is preferably set to be not less than 1% of the ground contactwidth of the tire 2 and not greater than 7% thereof. From the viewpointthat drainage and stiffness of the tread 4 are assured and inconsideration of the thickness of the tread 4, the depth of the maingroove 50 is set to be not less than 8.0 mm and not greater than 22.0mm. That is, the depth of the main groove 50 is not less than 8.0 mm andnot greater than 22.0 mm.

In the present invention, the ground contact width of the tire 2 isrepresented as the maximum width, in the axial direction, of the groundcontact surface. In order to obtain the ground contact width, the groundcontact surface is confirmed as follows. The tire 2 is mounted on anormal rim and the tire 2 is inflated with air to a normal internalpressure. The camber angle is set to 0°, and a normal load is applied tothe tire 2, and the tire 2 is brought into contact with a flat surface.Thus, the ground contact surface is obtained. The ground contact widthdescribed above is measured on the basis of the ground contact surface.

In the description herein, the normal rim represents a rim that isspecified according to the standard with which the tire 2 complies. The“standard rim” in the JATMA standard, the “Design Rim” in the TRAstandard, and the “Measuring Rim” in the ETRTO standard are included inthe normal rim.

In the description herein, the normal internal pressure represents aninternal pressure that is specified according to the standard with whichthe tire 2 complies. The “maximum air pressure” in the JATMA standard,the “maximum value” recited in “TIRE LOAD LIMITS AT VARIOUS COLDINFLATION PRESSURES” in the TRA standard, and the “INFLATION PRESSURE”in the ETRTO standard, are included in the normal internal pressure.

In the description herein, the normal load represents a load that isspecified according to the standard with which the tire 2 complies. The“maximum load capacity” in the JATMA standard, the “maximum value”recited in “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” in theTRA standard, and the “LOAD CAPACITY” in the ETRTO standard, areincluded in the normal load.

In the present invention, unless otherwise specified, the dimensions andangles of the components of the tire 2 are measured in a state where thetire 2 is mounted on a normal rim, and inflated with air to a normalinternal pressure. During the measurement, no load is applied to thetire 2.

In the tire 2, the tread 4 has three main grooves 50, in total, whichinclude a not-illustrated main groove 50. By the three main grooves 50,the tread 4 of the tire 2 has four ribs 48 formed therein.

As shown in FIG. 1, in the tire 2, the main groove 50 is disposed at theposition of the equator plane. In other words, the rib 48 is notdisposed at the position of the equator plane. In the tire 2, ribs 48 e,among the four ribs 48, disposed near the equator plane, are eachreferred to as a crown rib. Ribs 48 s disposed on the outer side in theaxial direction are each referred to as a shoulder rib. Further, maingrooves 50 s disposed near the shoulder ribs 48 s are each referred toas a shoulder main groove. In a case where the tread 4 has the oddnumber of ribs 48, the rib 48 is disposed at the position of the equatorplane in general. Therefore, in this case, the rib 48 disposed at theequator plane is referred to as the crown rib. In the present invention,the rib 48 e disposed at the equator plane of the tire 2 or at aposition near the equator plane is a crown rib, and the ribs 48 sdisposed on the outer side in the axial direction are shoulder ribs. Themain grooves 50 s disposed near the shoulder ribs 48 s are shoulder maingrooves. In the tire 2, outer surfaces 52 of the crown ribs 48 e andouter surfaces 54 of the shoulder ribs 48 s form a part of the treadsurface 28.

When the tire 2 is produced, a plurality of rubber members are assembledto obtain a raw cover (unvulcanized tire). The raw cover is put into amold (not shown). The outer surface of the raw cover contacts with acavity surface of the mold. The inner surface of the raw cover contactswith a bladder or a rigid core. The raw cover is pressurized and heatedin the mold. A rubber composition of the raw cover flows by thepressurizing and heating. Cross-linking reaction is caused in the rubberby the heating, to obtain the tire 2 shown in FIG. 1. By using a moldhaving an uneven pattern in the cavity surface, an uneven pattern isformed in the tire 2.

FIG. 2 shows a part of an outline of an outer surface 56 of the tire 2shown in FIG. 1. In FIG. 2, the up-down direction represents the radialdirection of the tire 2, the left-right direction represents the axialdirection of the tire 2, and the direction perpendicular to the surfaceof the drawing sheet represents the circumferential direction of thetire 2. The outline represents the shape of the outer surface 56 on thecross-section of the tire 2 along the plane that includes the centeraxis of the tire 2.

In the present invention, the outline of the outer surface 56 of thetire 2 is specified on the basis of the cavity surface of the mold. Theoutline, shown in FIG. 2, of the outer surface 56 of the tire 2corresponds to the shape of the cavity surface of the mold for the tire2. In other words, a part of the cavity surface of the mold used forproducing the tire 2 is shown in FIG. 2.

In FIG. 2, reference character PW represents the axially outer side endof the tire 2. A double-headed arrow WW represents a distance, in theaxial direction, from one of the outer side ends PW to the other of theouter side ends PW (not shown). The distance WW represents the width, inthe axial direction, of the tire 2. The outline of the outer surface 56of the tire 2 has the maximum axial width WW at the outer side ends PW.In other words, the outline of the outer surface 56 of the tire 2 hasthe maximum width WW. A double-headed arrow WT represents a distance, inthe axial direction, from one of the ends PT of the tread surface 28 tothe other of the ends PT (not shown) of the tread surface 28. Thedistance WT represents the width, in the axial direction, of the treadsurface 28.

In the tire 2, a ratio of the width WT of the tread surface 28 to themaximum width WW is not less than 0.7 and not greater than 0.9. Thus, inthe tire 2, a contact area can be assured and deformation can be madeappropriate in a well-balanced manner.

In the tire 2, the outline of the outer surface 52 of the crown rib 48 eis represented as an arc (hereinafter, referred to as a first arc). InFIG. 2, an arrow R1 represents the radius of the first arc. In the tire2, the radius R1 of the first arc is set to be not less than 300 mm andnot greater than 1000 mm. The range of the radius R1 is equivalent tothe range of the radius of an arc, which is set when, in a conventionaltire, the outline of the outer surface of the crown rib is representedas the arc. The center of the circle to which the first arc belongs isdisposed on the equator plane, which is not shown in FIG. 2 since theradius R1 is large.

In FIG. 2, reference character R1S represents one end (hereinafter,referred to as a first end) of the first arc. Reference character R1Erepresents the other end (hereinafter, referred to as a second end) ofthe first arc. A double-headed arrow L1 represents a distance from thefirst end R1S to the second end R1E. The distance L1 represents thelength of the chord of the first arc.

In the tire 2, in consideration of the size of the tire, stiffness ofthe crown rib 48 e, influence of the crown rib 48 e on performance ofthe tire 2, and the like, a length L1 of the chord of the first arc isset. Specifically, a ratio of the length L1 of the chord of the firstarc to the width WT of the tread surface 28 is preferably not less than0.1 and preferably not greater than 0.3. The ratio is more preferablynot less than 0.15 and more preferably not greater than 0.25.

In the tire 2, the outline of the outer surface 54 of the shoulder rib48 s is represented as an arc (hereinafter, referred to as a secondarc). In FIG. 2, an arrow R2 represents the radius of the second arc.Reference character R2S represents one end (hereinafter, referred to asa first end) of the second arc. Reference character R2E represents theother end (hereinafter, referred to as a second end) of the second arc.In the tire 2, the second end R2E is also the end PT of the treadsurface 28. A double-headed arrow L2 represents a distance from thefirst end R2S to the second end R2E. The distance L2 represents thelength of the chord of the second arc. Reference character CA representsthe middle point of the second arc, and reference character CSrepresents the center of the chord of the second arc. A double-headedarrow V represents a distance from the center CS of the chord of thesecond arc to the middle point CA of the second arc. The distance Vrepresents the distance from the center CS of the chord of the secondarc to the second arc, that is, represents an amount of projection.

In the tire 2, in consideration of the size of the tire, the stiffnessof the shoulder rib 48 s, influence of the shoulder rib 48 s onperformance of the tire 2, and the like, a length L2 of the chord of thesecond arc is set. Specifically, a ratio of the length L2 of the chordof the second arc to the width WT of the tread surface 28 is preferablynot less than 0.1 and preferably not greater than 0.3. The ratio is morepreferably not less than 0.15 and more preferably not greater than 0.25.

In the tire 2, a ratio of the distance V from the center CS of the chordof the second arc to the second arc relative to the length L2 of thechord of the second arc is not less than 0.03 and not greater than 0.05.When the ratio is set to be not less than 0.03, the outer surface 54 ofthe shoulder rib 48 s is prevented from being excessively flattened. Inthe tire 2, the outer surface 54 of the shoulder rib 48 s is preventedfrom being warped during cornering, and contact pressure at the firstend R2S and the second end R2E of the shoulder rib 48 s is appropriatelymaintained. In the tire 2, wear at the first end R2S and the second endR2E is effectively inhibited. Contact pressure at the middle point CA ofthe shoulder rib 48 s is prevented from being reduced, therebysufficiently obtaining cornering power. In the tire 2, in cornering,steering stability is good. When the ratio is set to be not greater than0.05, a difference between the circumferential length at the first endR2S and the circumferential length at the second end R2E isappropriately maintained in the shoulder rib 48 s. The extent to whichthe shoulder rib 48 s slips due to a difference in circumferentiallength is reduced so as to be small. Therefore, the shoulder rib 48 s isless likely to be worn due to the difference in circumferential length.Further, in cornering, the contact area in the shoulder rib 48 s can besufficiently assured. Therefore, in the tire 2, good steering stabilityis maintained.

As described above, in the tire 2, the shape of the outer surface 54 ofthe shoulder rib 48 s is appropriately adjusted by the length L2 of thechord of the second arc that represents the shape of the outer surface54, and the distance V from the center CS of the chord of the second arcto the second arc. In the shoulder rib 48 s, not only progress of weardue to the difference in circumferential length but also warping of theouter surface during cornering as observed in a conventional tire isinhibited. In the tire 2, steering stability is advantageously obtainedin cornering without reducing wear resistance. According to the presentinvention, the pneumatic tire 2 that allows steering stability to beimproved without reducing wear resistance can be obtained.

In the tire 2, a ratio of the distance V to the maximum width WW ispreferably not less than 0.005 and preferably not greater than 0.008.When the ratio is set to be not less than 0.005, the outer surface 54 ofthe shoulder rib 48 s is effectively prevented from being excessivelyflattened. In the tire 2, the outer surface 54 of the shoulder rib 48 sis prevented from being warped during cornering, and contact pressure atthe first end R2S and the second end R2E of the shoulder rib 48 s isappropriately maintained. In the tire 2, wear at the first end R2S andthe second end R2E is more effectively inhibited. Contact pressure atthe middle point CA of the shoulder rib 48 s is prevented from beingreduced, thereby obtaining more sufficient cornering power. In the tire2, in cornering, better steering stability is obtained. When the ratiois set to be not greater than 0.008, a difference between thecircumferential length at the first end R2S and the circumferentiallength at the second end R2E is appropriately maintained in the shoulderrib 48 s. The extent to which the shoulder rib 48 s slips due to thedifference in circumferential length is reduced so as to be smaller.Therefore, the shoulder rib 48 s is less likely to be worn due to thedifference in circumferential length. Further, in cornering, the contactarea in the shoulder rib 48 s is more sufficiently assured. Therefore,in the tire 2, good steering stability is maintained.

In the tire 2, a ratio of the radius R2 of the second arc thatrepresents the outline of the outer surface 54 of the shoulder rib 48 srelative to the radius R1 of the first arc that represents the outlineof the outer surface 52 of the crown rib 48 e is preferably not lessthan 0.15 and preferably not greater than 0.35. When the ratio is set tobe not less than 0.15, a difference between the circumferential lengthat the first end R2S and the circumferential length at the second endR2E is appropriately maintained in the shoulder rib 48 s. The extent towhich the shoulder rib 48 s slips due to the difference incircumferential length is reduced so as to be small. Therefore, theshoulder rib 48 s is less likely to be worn due to the difference incircumferential length. Further, in cornering, the contact area in theshoulder rib 48 s is sufficiently assured. Therefore, in the tire 2,good steering stability is maintained. In this viewpoint, the ratio ismore preferably not less than 0.20. When the ratio is set to be notgreater than 0.35, the outer surface 54 of the shoulder rib 48 s isprevented from being excessively flattened. In the tire 2, the outersurface 54 of the shoulder rib 48 s is prevented from being warpedduring cornering, and contact pressure at the first end R2S and thesecond end R2E of the shoulder rib 48 s is appropriately maintained. Inthe tire 2, wear at the first end R2S and the second end R2E iseffectively inhibited. Contact pressure at the middle point CA of theshoulder rib 48 s is prevented from being reduced, thereby sufficientlyobtaining cornering power. In the tire 2, in cornering, steeringstability is good. In this viewpoint, the ratio is more preferably notgreater than 0.30.

In FIG. 2, a solid line E represents an extension line of the secondarc. The extension line E is an arc that extends axially inward from thefirst end R2S of the second arc. Reference character R2A represents apoint of intersection of the extension line E and a line that representsthe outline of the side wall of the shoulder main groove 50 s.

As shown in FIG. 2, in the tire 2, the point R2A of intersection isdisposed radially inward of the first end R2S of the second arc. In thetire 2, in particular, increase of contact pressure at the first end R2Sof the shoulder rib 48 s is effectively inhibited. Also by the shape ofthe outer surface 54 of the shoulder rib 48 s being appropriatelyadjusted by the length L2 of the chord of the second arc that representsthe shape of the outer surface 54, and the distance V from the center CSof the chord of the second arc to the second arc, wear at the first endR2S is effectively inhibited in the tire 2. Further, the outer surface54 of the shoulder rib 48 s is prevented from being warped, wherebycontact pressure at the middle point CA of the shoulder rib 48 s isprevented from being reduced. An area of a portion in which the contactpressure is high is sufficiently assured, thereby sufficiently obtainingcornering power. The tire 2 exhibits excellent responsiveness, whereby adriver need not set a steering angle so as to be great by sharplyturning a steering wheel in cornering. The vehicle can sufficientlyperform cornering by a small steering angle, whereby a region of aportion, of the tire 2, which slips relative to a road surface isminimized. Therefore, the tire 2 is further inhibited from being worn.In the tire 2, steering stability is made good in cornering withoutreducing wear resistance. In this viewpoint, in the tire 2, the pointR2A of intersection of: the extension line E, of the second arc, whichextends axially inward from the first end R2S of the second arc; and theside wall of the shoulder main groove 50 s, is preferably disposedinward of the first end R2S in the radial direction.

EXAMPLES Example 1

A tire (tire size=205/85R16) shown in FIG. 1 was produced. When the tirewas produced, a mold having the cavity surface shaped as shown in FIG. 2was used. As a condition for a vulcanizer or the like for producing thetire, a condition for a conventional tire was used.

As indicated below in Table 1, in example 1, the length L2 of the chordof the second arc representing the outline of the outer surface of theshoulder rib was 33.8 mm. The maximum width WW of the tire was 198 mm.The distance V from the center CS of the chord of the second arc to thesecond arc, that is, the amount V of projection was 1.35 mm. Therefore,a ratio (V/L2) of the amount V of projection to the length L2 was 0.040.A ratio (V/WW) of the amount V of projection to the maximum width WW was0.007.

Comparative Example 1

A tire of comparative example 1 was obtained in the same manner as inexample 1 except that the distance V was 0 mm, that is, the outersurface of the shoulder rib was flat, and the ratio (V/L2) and the ratio(V/WW) were as indicated below in Table 1.

Examples 2 to 5 and Comparative Example 2

Tires of examples 2 to 5 and comparative example 2 were each obtained inthe same manner as in example 1 except that the radius R2 of the secondarc was different, and the amount V of projection, the ratio (V/L2), andthe ratio (V/WW) were as indicated below in Tables 1 and 2.

Wear Resistance

The tires were each mounted on a rim (16×8.5 J), and inflated with airto an internal pressure of 600 kPa. The tires were mounted to a 3-tontruck. The cargo bed was loaded with cargo such that a load of 12.6 kNwas applied to one tire. Thus, a state where the truck was loaded withits maximum load of the cargo was produced. A driver was caused to drivethe truck in a circuit at a speed of 80 km/h. After 100000 km running,an amount of wear of the tire was measured. The result is indicatedbelow as indexes in Tables 1 and 2. The greater the value of the indexis, the less the progress of the wear is, that is, the better the resultis.

Steering Stability

The tires were each mounted on a rim (16×8.5 J), and inflated with airto an internal pressure of 600 kPa. The tires were mounted to a 3-tontruck. The cargo bed was loaded with a cargo such that a load of 12.6 kNwas applied to one tire. Thus, a state where the truck was loaded withits maximum load of the cargo was produced. A driver was caused to drivethe truck in a circuit, and to evaluate steering stability. The resultis indicated below as indexes in Tables 1 and 2. The greater the valueof the index is, the more excellent steering stability is, that is, thebetter the result is.

TABLE 1 Evaluation result Compa. Example 1 Example 2 Example 3 Example 1L2 [mm] 33.8 33.8 33.8 33.8 WW [mm] 198 198 198 198 V [mm] 0.00 1.001.20 1.35 V/L2 [—] 0 0.030 0.036 0.040 V/WW [—] 0 0.005 0.006 0.007 Wear100 95 100 105 resistance Steering 100 105 110 110 stability

TABLE 2 Evaluation result Compa. Example 4 Example 5 Example 2 L2 [mm]33.8 33.8 33.8 WW [mm] 198 198 198 V [mm] 1.50 1.70 2.00 V/L2 [—] 0.0440.050 0.059 V/WW [—] 0.008 0.009 0.010 Wear 105 100 90 resistanceSteering 110 105 90 stability

As indicated in Tables 1 and 2, the evaluation is higher in the tires ofexamples than in the tires of comparative examples. The evaluationresult clearly indicates that the present invention is superior.

The technique for ribs described above is applicable to various tires.The foregoing description is in all aspects illustrative, and variousmodifications can be devised without departing from the essentialfeatures of the invention.

What is claimed is:
 1. A pneumatic tire comprising a tread having anouter surface that is a tread surface that comes into contact with aroad surface, wherein the tread includes a plurality of ribs that extendin a circumferential direction, and are aligned in an axial direction, arib, among the ribs, disposed at an equator plane of the tire, or nearthe equator plane, is a crown rib, and a rib, among the ribs, disposedon an outer side in the axial direction is a shoulder rib, an outersurface of the crown rib and an outer surface of the shoulder rib form apart of the tread surface, on a cross-section of the tire along a planethat includes a rotation axis of the tire, a ratio of a width, in theaxial direction, of the tread surface relative to a maximum width of thetire is not less than 0.7 and not greater than 0.9, an outline of theouter surface of the crown rib is represented as a first arc, and acenter of a circle to which the first arc belongs is disposed on theequator plane, an outline of the outer surface of the shoulder rib isrepresented as a second arc, and a ratio of a distance from a center ofa chord of the second arc to the second arc relative to a length of thechord of the second arc is not less than 0.03 and not greater than 0.05.2. The pneumatic tire according to claim 1, wherein a ratio of thedistance from the center of the chord of the second arc to the secondarc relative to the maximum width of the tire is not less than 0.005 andnot greater than 0.008.